The present invention relates to the field of cinema sound systems and more particularly it relates to structure of a compression-driven high-frequency loudspeaker module with improved waveguide structure and beamwidth compensation for deployment behind a perforated cinema screen.
In general, in a cinema loudspeaker system the object and the challenge to designers is to provide uniform coverage as perceived at all seating locations in the theater with regard to both loudness and flatness of frequency response, while causing the perceived sound source to coincide acceptably with the images projected on the screen.
A total screen array may utilize two or more speaker systems, typically xe2x80x9cstacksxe2x80x9d, located side-by-side (e.g. left, center and right); each of the stacks may be a three-way full frequency range unit having high-frequency, midrange and woofer portions. Typically, three multi-channel side-by-side arrays (left, center and right) each receive a unique directional signal with the object of recreating a left-to-right xe2x80x9csound stagexe2x80x9d as accurately as possible
FIG. 1 is graph showing a typical horizontal directivity index for a high quality conventional cinema high-frequency horn speaker as measured in free space, with no screen present. The curve shown depicts the target requirement that the frequency response of the directivity index should be held essentially flat over the high-frequency range (500 Hz-15 kHz). Directivity index expresses the gain (10 dB) in the peak direction on-axis, referred to an omnidirectional source having the same radiated power. and implies a corresponding beamwidth.
FIG. 2 is a graph showing the corresponding target value of 100 for horizontal beamwith measured at xe2x88x926 dB relative to the on-axis peak, as being 100 degrees total i.e +/xe2x88x9250 degrees from the axis. As in FIG. 1 for directivity, the objective is to hold the beamwidth constant over the frequency range.
Typically for cinema loudspeakers the vertical directivity pattern is made even more directional for coverage and efficiency: typical beamwidths are about 90 to 100 degrees horizontal by about 40 to 60 degrees vertical.
Traditionally cinema loudspeakers have been designed and developed to approach the target requirements shown in FIGS. 1 and 2 as nearly as possible, when tested alone without any intervening cinema screen present.
The majority of motion picture exhibitors locate the loudspeaker system behind a perforated vinyl screen in order to preserve accuracy of sound sourcing. The small diameter holes and resulting low ratio of open area combine to affect sound propagation.
In the graphs of FIGS. 3 and 4 the curves shown represent the frequency response of the directivity index and the beamwidth of a conventional cinema high-frequency loudspeaker originally designed and developed to perform as shown in FIGS. 1 and 2, when the loudspeaker is deployed behind a perforated screen. In FIG. 3, at high frequencies above about a few kHz the directivity index reduces with increasing frequency up to about 10 Khz, where it has dropped from 10 dB to about 6 dB, representing a substantial loss of sound level (4 dB). The corresponding beamwidth, shown in FIG. 4, increases due to the spreading caused by the cinema screen to about 140 degrees at 8 kHz and about 190 degrees at 16 kHz, compared to the target value of 100 degrees.
These primary changes due to the screen affect the actual performance as a loss of high-frequency sound level for most or all of the audience as excessive spreading of the high-frequency coverage pattern wastes sound energy in unwanted directions and regions, e.g. the theater side walls where it is dissipated as a combination of unwanted reflection and/or absorption.
This effect of the screen spreading the high frequencies has been observed as a shortcoming in evaluating and analyzing theater sound installations. It can sometimes be alleviated by adjusting the equalization for more power at the high frequencies, however often the amount of addition power is unavailable, so that in general this effect has remained an unsolved problem in the field of cinema sound systems and loudspeakers of known art.
It is a primary object of the present invention, in the design of a high-frequency loudspeaker for deployment behind a perforated cinema screen, to provide compensation for the effects of the screen perforations that decrease the sound pressure level and that spread the coverage pattern at the high-frequency end of the audio frequency spectrum.
It is a further object to accomplish defined coverage for all high frequencies with the loudspeaker deployed behind the screen.
It is still further object to provide a high-frequency cinema loudspeaker that is compensated for deployment behind a screen and that can be readily integrated in an array with a midrange portion of a cinema sound system.
The abovementioned objects have been accomplished in the present invention of a high-frequency cinema loudspeaker that is configured with a waveguide to compensate for the beam-spreading effects of a perforated cinema screen at the high end of the frequency range and to facilitate and coordinate integration with the other elements of a total sound system in providing defined coverage in a theater.
The specially shaped waveguide provides uniform sound directivity for high frequencies into the audience area by narrowing the horizontal coverage at the higher frequencies affected by the screen. The waveguide shape also provides an asymmetric dispersion pattern in the vertical plane to project the main axis of energy downwardly by an angle that is selected to optimize audience coverage.